The present invention relates to an automatic focusing method for scanning electron microscopy.
In a conventional scanning electron microscopy, the automatic focusing method has been attained in accordance with the following steps:
1) storing the secondary electron signals obtained by scanning electron beams at a constant speed in circular or linear scanning mode and further by varying the excitation condition of an objective lens at a predetermined rate; PA1 2) obtaining either one of an integration value of variations in signal between adjacent pixels on a displayed picture or an addition of absolute values of differences in signal between adjacent pixels, on the basis of the stored secondary electron signals; and PA1 3) obtaining an excitation condition of the objective lens in which the above-mentioned obtained value is maximized, to set the obtained excitation condition to the objective lens. (Set a central value of the excitation value width determined by a half of the maximum value to the objective lens by regarding the central value as the excitation condition in which the real maximum value can be obtained.)
In the above-mentioned conventional automatic focusing method since the electron beams are scanned at a constant speed in the circular or linear mode, where the boundary region (taper portion) of an object to be observed is located on the beam scanning lines, the components of topographic contrast are included to that extent in the obtained secondary electron signals, with a result that the automatic focusing precision is relatively high. In addition, when the beam scanning direction is perpendicular to the boundary region, it is possible to further improve the automatic focusing precision.
However, an object to be observed is generally located at any given position on the surface upon which the beams are irradiated. Therefore, in the case of the circular or linear scanning mode, the number of beams for scanning the boundary region of an object to be observed is not constant in the automatic focusing operation. Additionally, the topography of the object to be observed is not uniform, and further the boundary region is not always located along the direction perpendicular to the beam scanning direction. Accordingly, the topographic contrast components included in the obtained secondary signal fluctuate, thus resulting in a problem in that the precision of the automatic focusing operation is not stable.
Further, in the conventional automatic focusing method, since the automatic focusing operation is effected at a constant magnification (previously set by an operator usually), in case where the working distance between the objective lens and an object to be observed is changed markedly, there exists a problem in that the precision of the automatic focusing operation is often deteriorated.